lockdep: Print a nicer description for simple deadlocks

Lockdep output can be pretty cryptic, having nicer output
can save a lot of head scratching. When a simple deadlock
scenario is detected by lockdep (lock A -> lock A) we now
get the following new output:

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&(lock)->rlock);
  lock(&(lock)->rlock);

 *** DEADLOCK ***

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110421014259.643930104@goodmis.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 21 insertions, 0 deletions

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 73cebd7aa71..c4cc5d1acf4 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1664,6 +1664,26 @@ static inline void inc_chains(void)
 
 #endif
 
+static void
+print_deadlock_scenario(struct held_lock *nxt,
+                             struct held_lock *prv)
+{
+        struct lock_class *next = hlock_class(nxt);
+        struct lock_class *prev = hlock_class(prv);
+
+        printk(" Possible unsafe locking scenario:\n\n");
+        printk("       CPU0\n");
+        printk("       ----\n");
+        printk("  lock(");
+        __print_lock_name(prev);
+        printk(");\n");
+        printk("  lock(");
+        __print_lock_name(next);
+        printk(");\n");
+        printk("\n *** DEADLOCK ***\n\n");
+        printk(" May be due to missing lock nesting notation\n\n");
+}
+
 static int
 print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
                    struct held_lock *next)
@@ -1682,6 +1702,7 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
         print_lock(prev);
 
         printk("\nother info that might help us debug this:\n");
+        print_deadlock_scenario(next, prev);
         lockdep_print_held_locks(curr);
 
         printk("\nstack backtrace:\n");